Many persons who feed birds want to use a bird feeder that is selective as to the maximum size of a bird that is able to extract feed from a feeder. This limitation or selection thereby excludes larger size birds and animals, such as squirrels, which tend to frighten away smaller birds and consume quantities of feed in excess of that which a person desires to make available.
The above described problem or objective is known in the prior art and several patents disclose feeders which accomplish such objective in ways different from each other and from the invention described in detail hereinafter. U.S. Pat. No. 3,145,690--Bachman describes a bird feeder in which an inverted cylindrical hopper is suspended over a feeding pan so that feed from the hopper automatically fills the pan as the feed is consumed. A bird perch surrounds the pan and is attached to a conical shroud. Both the perch and the shroud are supported by a compression spring whereby heavier birds or animals on the perch cause the shroud to move downwardly and cover the feed pan.
U.S. Pat. No. 3,126,870--Matthew discloses a selective bird feeder in which a somewhat intricately shaped feed hopper has two side openings through which feed can be accessed. A cylindrical shroud surrounds the hopper and is mounted for rotation about a horizontal axis under the bias of a weight. The shroud has two openings normally aligned with the hopper access openings but which become misaligned, when a large bird perches on the shroud, to preclude access to the feed.
As well be apparent from the more detailed discussion below, the design of the bird feeder of the invention can be characterized as being very simple in shape with the advantage of being low cost to make or manufacture. Several patents disclose selective bird feeders which have more elaborate designs and operate in different ways. The following patents all show feeders having a box-like construction using some form of a shroud or shutter connected to a lever and which is actuated under the weight of a heavy bird to close access to feed: U.S. Pat. No. 3,104,649--Slaven, U.S. Pat. No. 3,124,103--Stainbrook, U.S. Pat. No. 3,241,525--Meier.
Also, the invention includes an inner feed hopper surrounded by a movable shroud which has openings therein that are normally aligned permitting access to the feed by lighter birds but which become misaligned to preclude access to the feed by heavier birds or animals. Two patents U.S. Pat. No. 1,404,251--Westenberger et al., and U.S. Pat. No. 3,738,329--Schweitzer describe bird (poultry) feeders that make use of aligned openings for controlling access to feed, but neither discloses using the weight of a bird to control the alignment of the openings. U.S. Pat. No. 1,404,251--Westenberger et al. shows a feeder having a handle attached to a shroud allowing the chicken raiser to open or close the feeder and control when the chickens will have access to the feed. U.S. Pat. No. 3,738,329--Schweitzer discloses a trough feeder having a movable plate provided with openings. The chicken raiser can position the plate to accommodate the growth of the size of a baby chick's head while preventing the whole chick from entering the feeding trough.